Portable air powered positive feed drills are used in industry to produce accurately placed and dimensioned holes in workpieces. One application of this type of drill is in the aircraft industry where the holes formed in the various aircraft parts must be located and drilled very accurately, often to within a tolerance of one-half of one-thousandth of an inch. In order to produce these accurate holes, it is necessary that the dimensions of the drill's cutter also be quite accurate. Once the cutter begins to dull, the dimensional tolerance of the hole it is producing can be exceeded solely because of deterioration of the cutter surface. In order to prevent this inaccuracy, it is a normal practice to change the drill cutters at regular intervals. However, it has been observed that where quantity production must be maintained operators may continue to use cutters that should be replaced, thereby completing a workpiece with holes, counterbores, and the like that are out of dimensional tolerance.
This invention provides a simple drill operation limiting device that signals an operator that the drill cutter should be changed. One embodiment of this invention prevents continued operation of the drill by blocking air flow to the air powered feed and retraction mechanism. Another embodiment of the invention provides an audible alarm in the form of a whistle that blows anytime the number of drilling operations has been exceeded.